a Precipitants are listed in order of preference: OH-, CO32- =base (sodium hydroxide or sodium carbonate), S2- = sulfide, SO42- = sulfate, and Cl- = chloride.
b The precipitant is for the indicated valence state.
c Very low maximum tolerance levels have been set for these low-toxicity ions in some countries, and large amounts should not be put into public sewer systems. The small amounts typically used in laboratories will not normally affect water supplies, although they may be prohibited by the local publicly owned treatment works (POTW).
d Recovery of these rare and expensive metals may be economically favorable.
e These ions are best precipitated as calcium molybdate(VI) or calcium tungstate(VI).
f CAUTION: Osmium tetroxide, OSO4, a volatile, extremely poisonous substance, is formed from almost any osmium compound under acid conditions in the presence of air. Reaction with corn oil or powdered milk will destroy it.
tion of 1 M sulfuric acid, or 1 M sodium hydroxide or carbonate. The pH can be determined over the range 1 through 10 by use of pH test paper.
The precipitate is separated by filtration, or as a heavy sludge by decantation, and packed for disposal. Some gelatinous hydroxides are difficult to filter. In such cases, heating the mixture close to 100 °C or stirring with diatomaceous earth, approximately 1 to 2 times the weight of the precipitate, often facilitates filtration.
As shown in Table 7.1, precipitants other than a base may be superior for some metal ions, such as sulfuric acid for calcium ion. For some ions, the hydroxide precipitate will redissolve at a high pH (Table 7.3). For a number of metal ions the use of sodium carbonate will result in precipitation of the metal carbonate or a mixture of hydroxide and carbonate.
7.D.3.8.3Chemicals in Which the Cation Presents a Relatively High Hazard from Toxicity
In general, waste chemicals containing any of the cations listed as highly hazardous in Table 7.1 can be precipitated as their hydroxides or oxides. Alternatively, many can be precipitated as insoluble sulfides by treatment with sodium sulfide in neutral solution (Table 7.4). Several sulfides will redissolve in excess sulfide ion, and so it is important that the sulfide ion concentration be controlled by adjustment of the pH.
Precipitation as the hydroxide is achieved as described above. Precipitation as the sulfide is accomplished by adding a 1 M solution of sodium sulfide to the metal ion solution, and then adjusting the pH to